7800 agp messing up help!

My friends 6800 gt died, so he decided to get a 7800. For some reason when he plays certain games like Call of Duty it works fine, but when he plays Call of Duty 2, and World of Warcraft there is massive polygon flicker, and world of warcraft a lot of times will freeze.

 -Already replaced it with another one.

-Just got a new power supply thats 450 watts

-Installed all of the new drivers, latest direct x

-reformatted hd

-Underclocked the card because maybe it was a heat issue, still having problems

-Reinstalled all of the games multiple times

What could be the problem? His 6800 gt worked fine.

Maybe he shouldn't be playing video games. Maybe he I mean I should be doin' Dishes!ShawtyChirp

[QUOTE="ShawtyChirp"]Maybe he shouldn't be playing video games. Maybe he I mean I should be doin' Dishes!JigglyWiggly2

This is his second card, and its doing the same thing. We underclocked the card with no luck, but it does only happen on some games. (COD 2 and WoW are the main problems) I researched more on Newegg about the card and it seems like a lot of people are having problems, some saying that the 7800 agp cards in general are quirky.

 So are their any other things to try without getting another card? Also...Would it be at all possible to exhange the card for a completely different one say maybe a 7600 gt through Newegg? Its only been a few days since he got the second card.